Elements of
Chemical Reaction Engineering
6th Edition



Home

Essentials of
Chemical Reaction Engineering
Second Edition

  Select  Chapter  >>     TOC     Preface     1     2     3     4     5     6     7     8     9     10     11     12     13     14     15     16     17     18     Appendices  
›

By Chapter Hide

  • Objectives
  • Living Example Problems
  • Extra Help
  • - Summary Notes
  • - LearnChemE Screencasts
  • - FAQs
  • - Interactive Computer Modules
  • Additional Material
  • Self Test
  • i>Clicker Questions
  • Professional Reference Shelf

By Concept Hide

  • Interactive Modules
  • -Web Modules
  • -Interactive Computer Games
  • Living Example Problems

U of M Hide

  • Asynchronous Learning
  • ChE 344
  • ChE 528

Chapter 10: Catalysis and Catalytic Reactors

Learning Resources

Example CD10-3: Decay in a Straight-Through Reactor

       
       
   

The gas-phase cracking of a Salina light gas-oil reaction

  
       

A typical cost
of the catalyst is
$1 million

  

gas-oil (g) products (g) + coke (s)
A B

  
   

is to be carried out in a straight-through transport reactor containing a catalyst that decays as a result of coking. The reaction is carried out at 750°F.

The entering concentration of A is 0.2 kmol/m 3 . The catalyst particles are assumed to move with the mean gas velocity (U g =U s = 7.5 m/s). The rate law is

  
       
   

image 10eq155.gif

 
       
   

with K A = 3 m 3 /kmol and K B = 0.01 m 3 /kmol. The maximum value of the term K B C B is small (0.002), so it can be neglected with respect to the other terms (e.g., 1). Using this fact and the bulk density,rhob , the rate law becomes

  
       
   

image 10eq156.gif

 
       
   

with k = 8 s -1 . At 750°F, the catalyst activity for light gas-oil over a synthetic catalyst for short contact times (i.e., less than 100 s) is

  
       
   

image 10eq157.gif

 
       
   

with A ´= 7.6 s -1/2 . As a first approximation, neglect volume change with reaction, pressure drop, and temperature variations.

Determine the conversion as a function of distance down the reactor. The reactor length is 6 m.

  
       
   




Solution

  
   

image 10eq158.gif

 
       
   

For a catalyst particle traveling with a velocity of U, the time that the catalyst particle has been in the reactor when it reaches a heightz is just

  
       
   

image 10eq159.gif

(CDE10-4.4)
       
   

and

  
       
   

image 10eq160.gif

(CDE10-4.5)
       
   

where

  
       
   

Stoichiometry. Gas-phase reaction withxi= 0, T = T 0 , and P = P 0.

  
   

image 10eq162.gif

 
       
   

Combining yields

  
       
   

image 10eq163.gif

(CDE10-4.6)
       
   

POLYMATH Solution. In using POLYMATH to obtain a solution, it is usually easiest to write the mole balance, rate law, and stoichiometry separately rather than combining them into a single equation. Therefore, starting with the mole balance
       
   

image 10eq164.gif

(CDE10-4.7)
       
   

we substitute for -r A and a using Equations (CDE10-4.2) and (CDE10-4.5). The POLYMATH program and solution are shown in Table CDE10-4.1 and Figure CDE10-4.1 respectively.

  
   

Table CDE10-4.1
POLYMATH Program Coking in a Straight-Through Transport Reactor

  
 

 
       
   


Figure CDE10-4.1
Conversion and Activity Profiles